1. Field of the Invention
This invention relates generally to systems for mechanically creating line- or pixel-based drawings piecemeal on a printing medium--whether the actual application of image to medium is performed by a laser-and-electrostatic, inkier, or dot-matrix process, or otherwise. More particularly the invention relates to mutually aligned printing of plural images, such as the several related images in a multicolor drawing, or in the set of masters made preliminary to printing a lithographic color separation.
For purposes of this document the term "printing medium" encompasses media which may be more familiarly regarded as only intermediaries--such as, for example, transparencies for use as lithographic masters.
2. Prior Art
Systems that create images piecemeal mechanically are subject to special registration problems arising from distortion and unintended shifting of the printing medium during the drawing process. Such problems are acute in large reel-to-reel drawing machines that very rapidly make multicolor drawings; some such drawings are tens of feet in length, but expected to be in intercolor register to just a few thousandths of an inch.
Even in smaller equipment, in systems that use liquid ink on paper, distortion of the paper can arise from application of the ink--as, for example, cockle of the paper. Still other systems are subject to like distortions arising from application of tension or toner, from environmental exposure of the medium, etc.
Thus each technology has its own sources of effective dislocation of each segment or portion of an image relative to others. Correction requires shifting of either the medium or the image on a substantially continuous basis, to maintain alignment between successive images--or, what amounts to the same thing, between each image and some sort of preestablished positional reference or coordinate grid on the medium.
Some prior systems control the position of printing for each segment of a drawing by reference to registration marks. These are typically preprinted alongside a previous single-color image--the marks and the image being formed together on the same medium. To correct for transverse expansion or contraction, and also for angular skew of the medium, marks sometimes are placed along two opposite sides of the image.
In some such systems, such as that disclosed in U. S. Pat. No. 4,721,969 to Asano (and in some of the prior art discussed in that patent), an essentially linear sensor array--such as a line of charge-coupled detectors (CCDs)--is employed to locate the edge of each registration mark in sequence. Assrio discusses locating the edges of respective marks preprinted along opposite sides of the image, so as to detect and so enable correction for angular skew as just mentioned.
In some related systems these functions may be performed in duplicate, to detect--for each mark--one edge in each of two mutually perpendicular directions. Such systems thereby locate the mark in two dimensions.
In most or all prior systems, each raster line from the sensor array is immediately processed to determine whether it reveals an encounter with a registration mark. If not, each line is immediately discarded (thereby wasting an opportunity to obtain useful information by correlation with subsequent lines).
The Assrio system is relatively advanced in that it makes corrections by software manipulations rather than by physically moving the printing medium or the writing mechanisms. His system is undesirable, however, in that he implements the software corrections by dropping or inserting a "bit"--presumably in most cases really a row of bits or pixels--to keep his writing mechanisms in synchronism with the preapplied registration marks. This procedure invites loss or distortion of image information.
(Asano does briefly suggest that instead "the drive during a recording operation for one line can be changed." He does not disclose how to do such a thing; it would place great demands upon the control system. Moreover, the idea of stopping the drive so as to superimpose one line or row of pixels on another--or advancing the drive to leave one line or row blank--would produce unacceptable image quality in the resulting striped effect.)
In prior systems several writing passes, for example four passes, may be used to create a four-color (typically cyan, magenta, yellow and black) picture. In this approach the first writing pass operates on an "open loop" basis--in other words, the apparatus is driven (without any servocontrol feedback) to produce a nominally regular spacing of elements that should be regularly spaced, etc.
Subsequent passes are servocontrolled, using feedback from the found positions of registration marks created in association with the first image. This represents an effort to align each segment of the later images with the corresponding segments of the first one.
It is known, however, as mentioned in U. S. Pat. No. 4,569,584 to St. John et al., that use of an open-loop first pass can lead to misalignment of the subsequent images with the first. This occurs because dynamic operation of the writing system in an open-loop mode diverges from dynamic operation under feedback control based upon the registration marks. Accordingly the first image--although in principle aligned more perfectly with the registration marks--is not as well aligned with the later images.
Therefore the St. John system and probably others instead provide for a first pass which preprints marks only, with substantially no other image. Thus instead of, for example, four passes to create a three-color-plus-black picture, such systems may use five passes--all of the last four being used to write onto the medium in a servocontrolled mode. (As St. John et al. mention, marks may instead be factory-preapplied to the printing medium. It is true that this raises the cost of the medium, but may be cost-effective in special environments with relatively small pieces of medium--such as, for example, transparency stock for use in making lithographic printing plates.)
In addition, some systems provide for a preliminary pass in which nothing is printed, but the medium is simply allowed to equilibrate with the temperature, humidity and perhaps chemical constituency of the environment. Such a preliminary pass is sometimes called a "conditioning pass".
The present document is equally applicable to refinement of all such methods. As will be seen, all these approaches suffer from a common problem.
In particular, as mentioned above, prior registration-mark detection systems find each mark by various techniques that detect the edge (or edges) of the mark. In effect such systems are intended to be--and substantially are--one-dimensional, with respect to each direction in which the mark edge is to be located.
That approach unavoidably provides very little data for each mark location; inherently, therefore, such a system is extremely sensitive to noise in the detection process. This is true whether the noise arises from imperfections in the preprinted mark or is developed in the detection system itself.
For instance the system will badly mislocate the mark if it was printed too lightly, or blurred, or with its leading edge (the edge closer to the direction from which the sensor system approaches) missing or indented. An indent or missing chunk along the edge, due for example to isolated imperfections in the surface of the printing medium or from occasional faulty operation of a writing mechanism, is sometimes called "flare".
In this field, however, terminology varies and is sometimes inconsistent. In particular, the term "flare" is sometimes used instead for opposite sorts of artifact in which an exploded dark area appears to be splashed from the registration mark.
For purposes of this document the term will be used more generally to describe both these effects--that is to say, both encroachment of whiteness from the white area into the mark, and encroachment of blackness outward from the mark into the surrounding white area. With respect to the latter, the term "flare" will be used to cover both discrete islands of black and attached protrusions from the mark.
Like problems occur even if the mark has a portion missing just inside the leading edge of the mark--in other words, enclosed within a solid line along the edge. This effect is usually called "dropout". (In our usage of the term "flare" described above, the generation of a discrete island of black can now be seen as the converse of "dropout".)
Dropout can be particularly troublesome if the missing internal portion is in a positional range within the field of a sensor that is pointed toward the leading edge. In such situations the sensor may average into its response the influence of the internal dropout region, and so may miss or mislocate the leading edge of the mark.
For two reasons, such flare and dropout problems are particularly significant for marks that are very small. First, with such marks, tiny imperfections can represent a large fraction of the signal along the leading edge.
Second, even the best available printing technologies often perform marginally when called upon to write a single pixel or a very small group of pixels. In particular such marginal operation is particularly likely to be erratic just after the apparatus is turned on, when it is not completely up to operating temperature.
Writing artifacts can also be generated in the pigment-delivery stage. In electrostatic systems, for example, optical-density variations can arise from incomplete development of the image by the electrically charged pigment--usually called "toner"--whether due to exhaustion of the toner supply or otherwise.
In addition, readers who are skilled in the field of electrooptical systems will appreciate that mark-sensor systems operate in the real world of vibration, airborne dust and surface-corrupting chemicals, electrical power-line disturbances, and electromagnetic interference from other electronic apparatus. Accordingly in the generation of mark-detection signals it is not economically feasible to eliminate occasional detection artifacts--both spurious indications of, and spurious failures to indicate, a mark or in particular a small element of a mark.
Hence all these effects militate in favor of registration marks that are relatively large, solid and geometrically regular--and therefore generally obtrusive. The larger, blacker and straighter the mark, the less sensitive to imperfections a one-dimensional sensor system may be. Such marks, however, may be described as "clubby".
They detract from the aesthetics of the finished product--the printed picture or diagram. They also therefore detract from the overall competitive appeal of the automatic drawing equipment. Even then, they do not adequately resolve the problems of sensitivity to flare, dropout, light or blurred printing, optical and electronic interference, etc.
A somewhat parallel discussion of signal-to-noise problems in registration-mark location appears in the St. John '584 patent at columns 33 and 34. As there suggested, a representative signal-to-noise ratio for operation of the mark-locating system is only 1/50.
The St. John system does have one feature that may be aimed at reducing sensitivity to edge effects: rather than detecting only one edge of each mark, St. John et al. arrange sensors or detectors to respond to either two opposed edges of the same mark, or the trailing edge of one mark and leading edge of the next. This feature is illustrated in FIG. 2, and FIGS. 8 through 10, of the '584 patent; and discussed in the corresponding text.
Those passages explain that the paired detectors or sensors in each arrangement are used to find a "balance" condition, in which the response from the two edges is equal. Consideration of this response regimen suggests that the result may be to partially average out the edge-imperfection effects, as between the two edges being sensed simultaneously.
For random artifacts this approach may be slightly beneficial, as it should halve the response to each artifact. This is not really a large enough factor, however, to obviate the fundamental problem, introduced earlier, of an insufficient amount of data to firmly establish the position of each mark.
The balanced-response technique, however, does introduce a complication in understanding the concepts of the present invention. For this reason we shall now discuss in some detail the precise nature of St. John's balanced dual-sensor configuration.
First, as already suggested, that system operates in substantially a one-dimensional mode, for its objective is to determine the position of an implicit line, or as St. John et al. put it a "transition" point. They refer to a transition between (1) a range of positions where one of the two sensors has higher response and (2) an adjacent range of positions where the other sensor has higher response.
As can be seen from the St. John illustrations, however, this system works because each detector has a noninfinitesimal width--i.e., dimension along the direction of edge determination. The drawings show that at the balance point each sensor is partly on and partly off its respective edge of its respective mark, or to put it otherwise each sensor overlaps the edge to which it responds.
Were it not for this nonzero sensor width, the St. John "balance" system could not operate. To understand the character and limitations of this system, however, we must explore carefully what happens if the sensor width is increased--or is decreased.
If each sensor were made wider, the fraction of its response due to the position of the mark edge would decrease. The sensitivity with which the system could find the "balance" or "transition" point would decrease accordingly.
If each sensor were made much wider, it would be able to receive optical signals from both edges of one mark at the same time. In other words, the whole mark would be sensed.
Such an arrangement would be entirely outside the operable invention of St. John et al., because small movements of the mark relative to the detector would not change the detector response at all. This excessive visibility would make regular operation impossible.
If on the other hand each sensor were made narrower, the sensitivity would increase. At certain points in the narrowing process, however, two adverse effects would set in.
One such effect is that the two sensors might no longer be able to both reliably overlap their respective mark edges at the same time. Taking into account unavoidable fluctuations in the spacing between marks, this effect must eventually appear.
Such fluctuations after all must be present, since it is the objective of the system to determine them. If this first effect does set in, the system will completely fail in its ability to find any balance point.
The other adverse effect is failure of the geometrical assumptions implicit in St. John's illustrations. A pure, infinitesimally thin line, as already noted, cannot "overlap" an edge (and of course also cannot receive nonzero amounts of optical energy)--but long before the sensor becomes that narrow its thickness must decrease to the order of the irregularities in the mark edge.
In other words, deviations of the mark edge from rectilinearity (or such other pattern as may be assumed) may be of the same magnitude as the sensor width. When this occurs, shifting the mark relative to the detector in either direction will produce compound effects, in which part of the mark edge moves out of range of the sensor even while part of the same edge is moving into range.
In such circumstances, no regularized monotonic response from the balance system is possible. Movement of the mark edge past the detector may produce reentrant behavior--waves, rather than steps--defying any systematic location of the desired "transition" or "balance" between the two successive marks.
Thus the width of each detector must be substantially greater than zero--and this could be interpreted to mean that, in one limited sense, the detector necessarily has the property of having area--but the width cannot be too great, certainly not as great as the width of the mark. A width value within a relatively tight optimum range is required.
In addition, as mentioned earlier, the system of St. John et al. depends upon a separate detector pair for locating each mark for each dimension, or in each direction. That is, even though in one sense each locating process relies on the detectors' possessing the property of area, nevertheless each locating process is only one-dimensional, not two-dimensional. Fundamentally St. John et al. are still using only the amount of information available in two edges--twice as much data or available "signal" as in one, but still not enough for entirely satisfactory measurement precision or, therefore, accuracy.
All of this may be summarized by saying that operation of the St. John dual-detector system depends upon pseudoareal effects requiring noninfinitesimal width of the detector. From the foregoing discussion it can be seen that some such effects are always present with any detector or sensor, and some are deliberately used in systems such as the St. John invention. The determinations involved, however, remain fundamentally one-dimensional.
The purpose of this extended discussion of the St. John system will become more clear shortly, upon introduction of tile present invention.
Some prior systems--including that presented in the St. John '584 patent--are made to average the found locations of several alignment-mark edges in sequence, and adjust the location of the image segment being printed in response to the averaged locations of the several mark edges. In such systems this is essential to avoid stepwise segmental displacements in the newly printed image--for example, a zigzag pattern in the image where a single mark edge was mistakenly "located" to one side of its actual position.
Such mark-edge averaging systems, however, introduce extremely undesirable effects. One of these is that they tend to correct at each point for positional errors first encountered several marks earlier.
That is, severe errors are allowed to accumulate rapidly without prompt correction. The St. John system averages, e.g., eight, sixteen or even thirty-two marks--as explained at columns 34 and 35 of the '584 patent--so that the system typically is correcting for errors actually found inches earlier.
(These relatively large numbers validate the comment made some twenty-one paragraphs above, to the effect that halving the response to edge effects, through St. John's "balanced" response technique, still does not provide sufficient data to enable reliable mark-location determinations. Again, despite its pseudoareal operating properties the St. John system fundamentally detects edge, or the balance or "transition" point between two edges; and even the information in two edges--though twice as much as the information in one--still is not enough to provide adequate signal-to-noise ratio or measurement accuracy.)
In an averaging system the actual trend of current data that are being corrected may even be the opposite of the average for a "sample group" on which the current correction is based. For example, using the numerical magnitudes suggested in the '584 patent, a sequence of sensor responses might be: 3 4 2 3 3 12 0 12 0 -1 -2 -1 -4 -4. The average of these sixteen readings is +9/16, which would round to +1 (calling for a correction of -1), but the clear-trend value for the last four responses is nearly -3. Hence the apparatus will apply a so-called "correction" of -1 pixel, even though the data are already off by -3 pixels--that is, the "correction" will shift the data even further in the wrong direction, away from its proper position.
Compensation for this effect can be attempted by positioning sensors farther upstream by, say, half the distance over which the several marks appear. This approach, however, at least in principle is undesirable because important distortions and shifts of the printing medium, etc., occur very close to the writing head. This consideration makes essential the placement of the sensors as close to the writing head as possible.
Perhaps a more-important adverse effect of a mark-edgedetection system is that good averaging is achieved only by associating a large spatial distance with the detection process. This expedient, however, inevitably also results in associating large measurement distances with each correction, which is undesirable because it degrades the correction process.
The present document is not a treatise on signal process-ing, and the skilled person in the art of assembling operational printing systems in general is not an expert in signal processing. As an aside, however, in the terminology of signal processing it can be appreciated that averaging techniques are simply one means of attempting to obtain a signal that is "well filtered".
The difficulty with these prior systems is that edge-detection techniques produce "poorly filtered" responses inherently. The reason is simple: an edge by its nature is intrinsically a transient phenomenon, and so by its nature is not well adapted to detection by "filtering" systems. This insight may be useful to a reader who happens to be familiar with the terminology.
St. John et al. concede (at their columns 34 and 35) many of the problems of averaging systems. They propose that optimizing the number of marks to be averaged in each "sample group" can obviate these problems, but as just explained the present inventors question such compromise solutions.
Despite all these very evident drawbacks of averaging systems, they are quite essential in all relevant prior-art registration systems because of the poor signal-to-noise ratio available in the mark-detection stage. Without averaging, as already suggested, prior-art systems would be susceptible to generating entirely unacceptable large, conspicuous lateral steps in the output data--that is, in each of the several images being created (supposedly in mutual register) on the printing medium.
Another feature disclosed by the St. John '584 patent is a continuously controlled mechanical device used to apply a correction in the ".theta." (theta) direction--i.e., an angular correction for skew of the medium relative to the apparatus. As can be appreciated such mechanical complexity is extremely troublesome and expensive, and best avoided if feasible.
One earlier electrostatic drawing system is of particular interest to the present inventors and their assignee, Hewlett Packard Company (HP) of Palo Alto, Calif. That system, sometimes identified as a Model 7600, was developed by HP in collaboration with Matsushita Graphics Commercial Systems (MGCS), a U. S. affiliate of the Japanese firm Matsushita.
Operation of the HP/MGCS system is good, but nevertheless susceptible to refinement. This system uses a CCD array--disposed across the entire width of the printing medium--to detect and localize both the longitudinal edges of the medium and the leading edge of each preprinted registration mark.
The edges of the printing medium, as located, are used automatically to make scan-axis (mainline, or x-axis) corrections. Accuracy of such measurements is limited by edge curl and light scattering--which have a tendency to cause the edges of the medium to appear out of focus to the CCD array.
The leading edges of the marks are used to correct for paper-axis (subline, or y-axis) corrections. Accuracy here is subject to the previously described inherent limitations of all edge-sensing systems. Both types of corrections are made on a software basis, which is greatly preferable to the expensive mechanical systems disclosed in the St. John et al. patent, but not by objectionably dropping or injecting bits (or introducing extraordinary drive demands) as in the Asano system.
Rather, the HP/MGCS system effects subline corrections by continuously controlling the speed of a motor which advances the printing medium, and mainline corrections by continuously controlling the lateral starting position of the recording head. This continuous-feedback regimen is advantageous as it maintains registration without losing image information--and. without artificially generating image elements that do not accurately represent input data.
Performance of this correction stage (as well as many other. portions) of the HP/MGCS system is of high quality. To retain this satisfactory performance, therefore, any refinement to overcome registration limitations should interface with as much as possible of the existing correction stage.
In all the earlier systems discussed above, ability to detect the mark edges is limited by artifacts, optical-density variations and translucent media as already set forth. The results of these errors can be color vectors offset by more than one pixel. In raster imaging such errors appear as hue shifts and checkerboard patterns.
To summarize, intrinsically all edge-detection systems, or interedge-transition systems, or "balance" systems, or any other fundamentally one-dimensional systems are intolerant of error from a great variety of sources. Whereas the term "robust" is currently in vogue to describe or characterize systems whose accuracy is relatively insensitive to perturbations, by contrast prior systems in the field of the present invention may be characterized as "frail".
Some systems avoid the entire matter of registration marks by fastening the printing medium to a drum or a like moving support. In such a system the location of each segment; of the medium is substantially fixed to a corresponding portion of the support.
Controlling the position of the support relative to the writing mechanism then can be taken as controlling the position of the medium relative to the writing mechanism, without the need for registration marks. Such systems, however, have their own severe drawbacks of mechanical size and complexity, cost, limited size of the printing medium, and so forth.
In other fields, not heretofore related to the plural-image printing industry, different approaches have been used for automatic locating or orienting processes. For example it is believed that in object identification for vision systems, for printed-circuit board alignment, and for industrial component identification, schemes are in use that entail pattern matching.
Signal pattern recognition is also used in even more-remote and more-abstract applications such as identifying radio-frequency transmissions or processing digital signals. As far as we are aware, however, none of such techniques has been suggested for use in alignment of images for printing.
Thus the prior art in this field has failed to provide robust techniques for locating registration marks and printing images in alignment with such marks.